SD Loader

Introduction

Legacy computer systems typically load and save programs over serial ports. This is often done by attaching a PC and running a terminal program to send/receive programs to the legacy systems. Integrating modern storage, like SD cards, can be a serious challenge into legacy designs.

Replacing the external PC offers the opportunity to more easily transport the system. A compact solution that emulates the PC for transfers would be useful.

This design transfers files to/from an SD card via a serial port. It runs a menu driven program on small OLED to control transfers and supports SD cards up to 32GB (FAT32 formatted). The serial interface has RTS/CTS hardware handshake support. It can run with 5V or 3.3V target systems or at RS-232 levels. Custom cards could easily be added to emulate cassette interfaces or run.

Features

Supports SD cards up to 32GB
- FAT32 formatted
Serial interface
- Full hardware Handshake support

Cards

Constructed out of existing Land Boards cards
QT Py board
- RP2040 board versions
  - SAMD board does not have enough SRAM to use CircuitPython, needs Arduino IDE
  - RP2040 board has a lot more SRAM and is easier to program for directory lists
- QTPy49 QT Py or XIAO RP2040 Adapter card
SD CARD X49
MyMenu
- MCP23008 I2C interface
- OLED
- 5 Pushbuttons
- 3 LEDs
Optional DCE or DTE RS-232 Level Translator card
Optional Level Shifter mounted on GRID49 card
- Used for legacy 5V system targets
Optional Mini360 Adapter Board
- Wide-range input Power card (7-23V)

QTPy49 Card

Easier to cable external cards
Flexible power (7-12V)

SD CARD X49

Build 3.3V version
No regulator
Install NO5V jumper

MyMenu

I2C interface to OLED, I2C Port Expander with 3 LEDs, 5 switches

MCP23008 Data Sheet

GPIO Port Map
GP0 - LED D3
GP1 - LED D2
GP2 - LED D1
GP3 - Select button 0 = pressed
GP4 - Right button
GP5 - Down button
GP6 - Up button
GP7 - Left button

RS-232 Level Shifter (Optional)

5V Level Shifter (Optional)

Use for "legacy" 5V Serial target systems
- Not used for 3.3V systems
Convert 3.3V QT Py Tx/Rx/RTS/CTS to "legacy" 5V
Isolated 3.3V/5V power
Common Ground
Built on GRID49 card
Adafruit 4-channel I2C-safe Bi-directional Logic Level Converter

JoyPad - Alternate to MyMenu card

JOYPAD
Used if PiPicoMite01 is used in place of QT Py

QTPy49 Card Cabling

QT Py Card has function specific headers

QTPy49 Card to MyMenu Cabling

QTPy49	SIGNAL	MyMenu	COLOR
J5-1	GND	P2-1	BLK
J5-2	+3.3V	P2-2	RED
J5-3	SDA	P2-3	WHT
J5-4	SCL	P2-4	BRN
J9-3	INT(AD0)	P2-5	GRY

QTPy49 Card to SD_CARD_X49 Cabling

QT Py	SIGNAL	SD_CARD_X49	COLOR
J3-1	GND	J1-1	BLK
J3-2	3.3V	J1-8	RED
J3-4 (MO)	MOSI	J1-4	WHT
J3-5 (MI)	MISO	J1-2	BRN
J3-6 (SCK)	SCK	J1-3	BLU
J6-3 (A3)	SS*	J1-5	GRY

QTPy49 Card to Host Serial

QTPy49	SIGNAL	COLOR
J8-3 (AD1)	RTS*	GRY
J7-1 (AD2)	CTS*	ORA
J4-4 (RX)	RX	BRN
J4-3 (TX)	TX	WHT
J4-2 (VCC)	+3.3V	Xlator (only)
J4-1 (GND)	GND	BLK

QT Py Breakout Card (Alternate)

Do not install 5mm Terminal Block

Power via USB C cable

QT Py Breakout Card (on GRID49 card) Cabling

GRID49 card
QT Py uses (2) 1x7 male headers for signals
- Cut off 1 set of holes of (2) female housings to make 1x7 female headers
SD Card uses (1) 1x8 header
MyMenu uses (1) 1x5 header
QT Py uses 2x8 header for 3.3V and Ground
- Use 2x3 female housings
40mm cables to Host Target

QT Py Breakout Card Cabling

QT Py	SIGNAL	CARD	PIN	COLOR
01 (AD0)	INT(AD0)	MyMenu	P2-5	GRY
02 (AD1)	RTS*	TARGET	TBD	GRY
03 (AD2)	CTS*	TARGET	TBD	ORA
04 (AD3)	SS*	SD_CARD	J1-5	GRY
05 (SDA)	SDA	MyMenu	P2-3	WHT
06 (SCL)	SCL	MyMenu	P2-4	BRN
07 (TX)	TX	TARGET	TBD	WHT
08 (RX)	RX	TARGET	TBD	BRN
09 (SCK)	SCK	SD_CARD	J1-3	BLU
10 (MI)	MISO	SD_CARD	J1-2	BRN
11 (MO)	MOSI	SD_CARD	J1-4	WHT
12 (3V)	+3.3V	GRID49	Q7	WIRE
13 (GND)	GND	GRID49	R7	WIRE
14 (5V)	5V	N/C
PWR-01	+3.3V	SD_CARD	J1-8	RED
PWR-02	+3.3V	TARGET	TBD	RED
PWR-03	+3.3V	MyMenu	P2-2	RED
PWR-14	GND	MyMenu	P2-1	BLK
PWR-15	GND	SD_CARD	J1-1	BLK
PWR-16	GND	TARGET	TBD	BLK

Arduino Software

i2cscan of attached cards shows OLED at 0x3c and MCP23008 at 0x20

I2C addresses found: ['0x20', '0x3c']

Arduino Software Development Environment

Developed using Arduino IDE V1.8.xx
Arduino IDE Setup for QT Py
Requires Arduino IDE version 1.8 or higher
Add to Preferences Additional Boards Manager URLs

https://adafruit.github.io/arduino-board-index/package_adafruit_index.json

pinMode(pin, INPUT_PULLUP)

#if defined(ARDUINO_SAMD_ZERO) && defined(SERIAL_PORT_USBVIRTUAL)
  // Required for Serial on Zero based boards
  #define Serial SERIAL_PORT_USBVIRTUAL
#endif

const char str[] = "My very long string";

UF2 Bootloader Details

Arduino Human Interface Design (HID)

Menu driven
- MyMenu card driver
OLED display
- ug2 u8x8 OLED display driver software

Arduino SD Card

Library
Example files

Arduino SD Card Library

SD Card library
- The SD library allows for reading from and writing to SD cards, e.g. on the Arduino Ethernet Shield
- Based on sdfatlib
- The library supports FAT16 and FAT32 file systems on standard SD cards and SDHC cards
- It uses short 8.3 names for files
- The file names passed to the SD library functions can include paths separated by forward-slashes, /, e.g. “directory/filename.txt”
- Because the working directory is always the root of the SD card, a name refers to the same file whether or not it includes a leading slash (e.g. “/file.txt” is equivalent to “file.txt”)
- As of version 1.0, the library supports opening multiple files.
- The communication between the microcontroller and the SD card uses SPI, which takes place on digital pins 11, 12, and 13 (on most Arduino boards) or 50, 51, and 52 (Arduino Mega)
- Additionally, another pin must be used to select the SD card
- This can be the hardware SS pin - pin 10 (on most Arduino boards) or pin 53 (on the Mega) - or another pin specified in the call to SD.begin()

Arduino SD Card Example Code

Arduino SD Card Connections

Uses default board SPI pins
Uses D3 as chip select

Serial

Serial Reference
Serial.begin(speed)
Serial.available()
- Get the number of bytes (characters) available for reading from the serial port.
- This is data that’s already arrived and stored in the serial receive buffer (which holds 64 bytes).
- Turn off/on receive handshake based on Serial.available() count and storage
Serial.write()
- Writes binary data to the serial port
- This data is sent as a byte or series of bytes
- To send the characters representing the digits of a number use the print() function instead.
- Syntax

Serial.write(val)
Serial.write(str)
Serial.write(buf, len)

CircuitPython Software

CircuitPython has several advantages over Arduino IDE
- Build-test-built cycle time much faster
- Much easier to do list processing for directory structure
- Speed not critical in this application
Not enough SRAM with SAMD version of the card
- RP2040 version of the card has plenty of SRAM
- XAIO RP2040 is $5.40 and ships quickly from US Warehouse

CircuitPython Software Development Environment

XIAO RP2040 with CircuitPython
Our GitHub repo
Use Thonny IDE
- Left click bottom right corner
- Select CircuitPython (generic)
- Left click bottom right corner
- Select Configure Interpreter
- Select USB Serial Device (COM<x)
Pins list

import board
dir(board)
['__class__', '__name__', 'A0', 'A1', 'A2', 'A3', 'D0', 'D1', 'D10', 'D2', 'D3', 'D4', 'D5', 'D6', 'D7', 'D8', 'D9', 'I2C', 'LED', 'LED_BLUE', 'LED_GREEN', 'LED_RED', 'MISO', 'MOSI', 'NEOPIXEL', 'NEOPIXEL_POWER', 'RX', 'SCK', 'SCL', 'SDA', 'SPI', 'TX', 'UART', 'board_id']

>>> print(board.board_id)
seeeduino_xiao_rp2040

Install CircuitPython on Seeed XIAO RP2040

Long press the "BOOT" button. (The "B" is written on the board shown to the right)
Connect the XIAO PR2040 to your computer while you still pressing the button.
The computer then will appear a disk driver(RP1-RP2)
Download the XIAO-RP2040-CircuitPython.uf2 file for XIAO RP2040
Drag the .uf2 file to the disk driver("RP1-RP2")
Check the disk drive if the name has changed to "CIRCUITPY" (takes a few seconds)
Free memory

>>> import gc
>>> gc.mem_free()
208128
>>> import time
>>> gc.mem_free()
207952

CircuitPython Human Interface Design (HID)

MyMenu card
- 0.91" OLED
- 3 LEDs
- 5 pushbuttons
i2cscan of attached cards shows OLED at 0x3c and MCP23008 at 0x21
Added i2cscan code to find the MCP23008 on the MyMenu card
MyMenu demo/test code
- Read pushbuttons
- Write OLED
- Write LEDs
- Exit by pressing Select button

CircuitPython SD Card

sdcardio.SDCard(bus: busio.SPI, cs: microcontroller.Pin, baudrate: int = 8000000)

Example code

Initialize SD card and read directory
Result

>>> %Run -c $EDITOR_CONTENT
Files on filesystem:
====================
System Volume Information/               Size:       0 by
   WPSettings.dat                        Size:      12 by
   IndexerVolumeGuid                     Size:      76 by
SuperStarTrek.bas                        Size:    20.0 KB
scramble.bas                             Size:    19.2 KB
CivilWar.bas                             Size:    13.5 KB
>>>

Storing path, file name as set pairs, gives

dirFileNames [('/sd', 'System Volume Information'), ('/sd/System Volume Information', 'WPSettings.dat'), ('/sd/System Volume Information', 'IndexerVolumeGuid'), ('/sd', 'SuperStarTrek.bas'), ('/sd', 'scramble.bas'), ('/sd', 'CivilWar.bas')]
('/sd', 'System Volume Information')
('/sd/System Volume Information', 'WPSettings.dat')
('/sd/System Volume Information', 'IndexerVolumeGuid')
('/sd', 'SuperStarTrek.bas')
('/sd', 'scramble.bas')
('/sd', 'CivilWar.bas')

CircuitPython Serial

busio UART reference
Supports RTS/CTS
- MicroPython UART flow control
Constructor

lassbusio.UART(tx: microcontroller.Pin, rx: microcontroller.Pin, *, baudrate: int = 9600, bits: int = 8, parity: Optional[Parity] = None, stop: int = 1, timeout: float = 1, receiver_buffer_size: int = 64)

RTS, CTS pins defines on QT Py RP2040
- RTS = pin D2
- CTS = pin D3

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